A number of liquefaction systems for cooling, liquefying, and optionally sub-cooling natural gas are well known in the art, such as the single mixed refrigerant (SMR) cycle, the propane pre-cooled mixed refrigerant (C3MR) cycle, the dual mixed refrigerant (DMR) cycle, C3MR-Nitrogen hybrid (such as AP-X™) cycles, the nitrogen or methane expander cycle, and cascade cycles. Typically, in such systems, natural gas is cooled, liquefied, and optionally sub-cooled by indirect heat exchange with one or more refrigerants. A variety of refrigerants might be employed, such as mixed refrigerants, pure components, two-phase refrigerants, gas phase refrigerants, etc. Mixed refrigerants (MR), which are a mixture of nitrogen, methane, ethane/ethylene, propane, butanes, and pentanes, have been used in many base-load liquefied natural gas (LNG) plants. The composition of the MR stream is typically optimized based on the feed gas composition and operating conditions.
The refrigerant is circulated in a refrigerant circuit that includes one or more heat exchangers and one or more refrigerant compression systems. The refrigerant circuit may be closed-loop or open-loop. Natural gas is cooled, liquefied, and/or sub-cooled by indirect heat exchange against the refrigerants in the heat exchangers.
Each refrigerant compression system includes a compression circuit for compressing and cooling the circulating refrigerant, and a driver assembly to provide the power needed to drive the compressors. The refrigerant compression system is a critical component of the liquefaction system because the refrigerant needs to be compressed to high pressure and cooled prior to expansion in order to produce a cold low pressure refrigerant stream that provides the heat duty necessary to cool, liquefy, and optionally sub-cool the natural gas.
A majority of the refrigerant compression in base-load LNG plants is performed by dynamic or kinetic compressors, and specifically centrifugal compressors, due to their inherent capabilities including high capacity, variable speed, high efficiency, low maintenance, small size, etc. Other types of dynamic compressors such as axial compressors and mixed flow compressors have also been used for similar reasons. Dynamic compressors function by increasing the momentum of the fluid being compressed. In contrast, positive displacement compressors function by reducing the volume of the fluid being compressed. Positive displacement compressors such as reciprocating and screw compressors have typically not been preferred in base-load LNG service because of their lower flow capability that in turn leads to the need for many units, higher cost, and larger plot area.
There are four main types of drivers that have been used in LNG service, namely industrial gas turbines, aero-derivative gas turbines, steam turbines, and electric motors.
In some scenarios, the LNG production rate may be limited by the installed refrigerant compressor. One such scenario is when the compressor operating point is close to the anti-surge line. Surge is defined as an operating point at which the maximum head capability and minimum volumetric flow limit of the compressor are reached. The anti-surge line is an operating point at a safe operating approach to surge. An example of such a scenario for a C3MR cycle is at high ambient temperature where there is an increased load on the propane pre-cooling system causing the maximum head and thereby lowest allowable flow rate to be reached. Therefore, the refrigerant flow rate is limited, which then limits the refrigeration and LNG production rate.
Another scenario where the LNG production rate is limited by the installed refrigerant compressor is when the compressor is close to stonewall or choke. Stonewall or choke is defined as the operating point where the maximum stable volumetric flow and minimum head capability of the compressor are reached. An example of such a scenario is when the plant is fully loaded and is running at maximum LNG capacity. The compressor cannot take any more refrigerant flow through it and the plant is therefore limited by the compressor operation.
A further scenario where the LNG production may be limited by the installed refrigerant compressor is for large base-load facilities where the compressor operating points are limited by compressor design specifications, such as the flow coefficient, the inlet Mach number, etc.
In some scenarios, the LNG production is limited by the available driver power. This can happen when the plant is operating at high LNG production rates. It can also happen for plants with gas turbine drivers at high ambient temperature due to reduced available gas turbine power.
One approach to debottleneck the refrigerant compression system is to add an additional dynamic compressor, such as a centrifugal compressor, with its driver at the discharge of the primary compressor. This helps build more head into the compression system for a scenario where the compressor is operating close to the anti-surge line, but adding an additional dynamic compressor at the discharge of the primary compressor has limited benefits when the compressor is operating close to stonewall. Therefore, the addition of the additional dynamic compressor will not solve the problem of maximum flow constraint.
Another approach has been to add a secondary dynamic compressor such as a centrifugal compressor in parallel with the primary compressor. The secondary compressor is typically much smaller in capacity as compared to the primary compressor and this poses a challenge with respect to balancing the two parallel compressors and ensuring that the outlet pressure match up although the volumetric flow rates may not. The head versus capacity curve of a typical dynamic compressor is shown in FIG. 1. Given the gradual shape of the curve, matching the head at the outlet yet making sure that the total flow adds up the desired refrigerant flow can be challenging. The addition of a similar size second compressor to debottleneck the system is not a likely option due to the large costs associated with matching the compressor size.
Furthermore, it is difficult to adjust the flow split between two parallel dynamic compressors with different flow characteristics (as described above) as operational conditions in the compression system change. For example, in a C3MR plant operating close to the anti-surge line, as the ambient temperature reduces, the approach to surge increases and a lower flow rate through the secondary compressor is required. Additionally, the parameters of the secondary compressor, such as speed, typically cannot be varied because such variation will result in a change in the outlet pressure, creating an imbalance with the primary compressor. Further, in scenarios wherein the primary compressor is a mixed refrigerant compressor, any variations in MR composition with changing feed composition and ambient conditions might lead to an imbalance of the two compressors. Many of these challenges are driven by the fact that both compressors are not identical and the second compressor is typically of much smaller capacity than the main compressor.
Overall, adding a lower capacity dynamic compressor in parallel with the primary compressor leads to an inflexible design that could be challenging to design and operate efficiently. Therefore, what is needed is a simpler and more efficient method of debottlenecking loaded compression systems in an LNG plant.